So I had planned to clean up the source code for SMS Popup before releasing the source code - however, it seems the cats out of the bag as people are stumbling upon the code already over at code.google.com:http://code.google.com/p/android-smspopup/
You should be able to download and build the app yourself without much issue. The code is still quite messy and if anyone spots bugs or issues feel free to file them on the code page (or ideally help me fix them hehe).
Hopefully this will help beginner Android developers wrap their head around working with SMS and MMS messages.
Update: I just noticed Mike DG, creator of the dgAlert app in the Market has just switched his app over to the SMS Popup code base.